---
title: "Explain the different types of keys used in the SQL table with examples."  
description: "Explain the different types of keys used in the SQL table with examples."  
author: "Revati S Misra"  
published: 2023-03-24  
updated: 2023-04-28  
canonical: https://www.mindstick.com/forum/157563/explain-the-different-types-of-keys-used-in-the-sql-table-with-examples  
category: "mssql server"  
tags: ["database", "mysql", "mssql server", "sql server", "sql"]  
reading_time: 4 minutes  

---

# Explain the different types of keys used in the SQL table with examples.

[Explain](https://www.mindstick.com/forum/157854/what-is-system-debugging-explain-some-system-debugging-tools-used-in-modern-computer-systems) the different types of key [constraints](https://www.mindstick.com/forum/34816/what-is-mapping-constraints-in-database-management-system) used in the [SQL table](https://www.mindstick.com/forum/160519/how-can-i-update-sql-table-by-dataset-data-in-ado-dot-net) with their examples.

## Replies

### Reply by Aryan Kumar

In SQL, keys are used to identify unique records in a table and establish relationships between tables. There are different types of keys used in SQL tables, including:

- **Primary Key:** A primary key is a unique identifier for each record in a table. It cannot contain null values and must be unique for each record. The primary key is used to create relationships between tables. Example:

```php
CREATE TABLE customers (
 id INT PRIMARY KEY,
 name VARCHAR(50),
 email VARCHAR(50)
);
```

In this example, the **id** column is the primary key for the **customers** table.

- **Foreign Key:** A foreign key is a field in one table that refers to the primary key in another table. It is used to establish relationships between tables. Example:

```php
CREATE TABLE orders (
 id INT PRIMARY KEY,
 customer_id INT,
 total_price DECIMAL(10,2),
 FOREIGN KEY (customer_id) REFERENCES customers(id)
);
```

In this example, the **customer_id** column in the **orders** table is a foreign key that refers to the **id** column in the **customers** table.

- **Unique Key:** A unique key is similar to a primary key in that it identifies unique records in a table, but it can contain null values. Example:

```php
CREATE TABLE products (
 id INT PRIMARY KEY,
 name VARCHAR(50),
 sku VARCHAR(20) UNIQUE,
 price DECIMAL(10,2)
);
```

In this example, the **sku** column is a unique key for the **products** table, which means that it can contain null values but must be unique for each record.

- **Composite Key:** A composite key is a combination of two or more columns that are used as a unique identifier for each record in a table. Example:

```php
CREATE TABLE order_items (
 order_id INT,
 product_id INT,
 quantity INT,
 PRIMARY KEY (order_id, product_id),
 FOREIGN KEY (order_id) REFERENCES orders(id),
 FOREIGN KEY (product_id) REFERENCES products(id)
);
```

In this example, the combination of the **order_id** and **product_id** columns is used as a composite primary key for the **order_items** table.

In summary, SQL tables use different types of keys to identify unique records, establish relationships between tables, and maintain data integrity. These keys include primary keys, foreign keys, unique keys, and composite keys.

### Reply by Krishnapriya Rajeev

In [SQL](https://www.mindstick.com/articles/13115/types-of-keys-in-sql-or-oracle-database), different types of keys are used to establish relationships between the tables and ensure integrity. Some of the commonly used keys include primary keys, foreign keys, unique keys, composite keys, and candidate keys.

1. **PRIMARY KEYS**: A primary key uniquely identifies each row in a [table](https://www.mindstick.com/articles/43918/how-to-design-table-using-bootstrap) and cannot contain null values. Each table can have only one primary key.

![Explain the different types of keys used in the SQL table with examples.](https://www.mindstick.com/mindstickforums/b6a03c20-73bf-447f-a1ee-ae523f2c1e07/images/5ce0202b-cb6d-4dfb-a8ac-9c94787bd995.png)

In this example, id column is the PRIMARY KEY.

2. **FOREIGN KEYS**: A foreign key is a type of column found in a table that utilizes the values present in the primary key of a separate table. The values in this column can either be an already existing primary key from the referenced table or null and cannot be anything else.

![Explain the different types of keys used in the SQL table with examples.](https://www.mindstick.com/mindstickforums/b6a03c20-73bf-447f-a1ee-ae523f2c1e07/images/6cabdbaa-4bf3-41c5-a2ea-4bd8789786bd.png)

In this example, emp_id acts as a foreign key in the table ‘DEPENDANTS’ that refers to the primary key of the table ‘EMPLOYEES’.

3. **UNIQUE KEYS**: A unique key is a column in a table that has a unique constraint applied to it. It can have null values but no two values can be the same.

![Explain the different types of keys used in the SQL table with examples.](https://www.mindstick.com/mindstickforums/b6a03c20-73bf-447f-a1ee-ae523f2c1e07/images/4c69b90d-6b60-46f5-8506-d532c5e9fb62.png)

In this example, ‘position’ column is a unique key.

4. **COMPOSITE KEYS**: A composite key consists of two or more columns that together uniquely identify each row in a table.

![Explain the different types of keys used in the SQL table with examples.](https://www.mindstick.com/mindstickforums/b6a03c20-73bf-447f-a1ee-ae523f2c1e07/images/6bd1a448-10a4-47e0-95b5-17a26281243a.png)

In this example, the composite key is made up of the ‘emp_id’ and ‘mobile’ columns in the ‘EMPLOYEES’ table.

5. **CANDIDATE KEY**: A candidate key is a column or a group of columns that could potentially be used as a primary key and must be unique and not contain null values.

![Explain the different types of keys used in the SQL table with examples.](https://www.mindstick.com/mindstickforums/b6a03c20-73bf-447f-a1ee-ae523f2c1e07/images/43c09159-1057-4967-ba97-cf5a8d553c85.png)

In this example, the ‘mobile’ column is a candidate key that could potentially be used as the primary key.


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Original Source: https://www.mindstick.com/forum/157563/explain-the-different-types-of-keys-used-in-the-sql-table-with-examples

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